DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention provides novel processes that use metal substituted xerogels to improve the delignification and brightening properties of kraft pulps, preferably in an alkaline peroxide bleaching stage. The use of metal substituted xerogels for improving alkaline peroxide delignification of kraft pulps has not been reported previously. Xerogels are zeolite-type structures, however, while zeolites have been shown to be ineffective in the bleaching of kraft pulps. The novel metal xerogel activated processes of the present invention surprisingly improve hydrogen peroxide bleaching of a variety of pulps. The term “bleaching” includes both decolorization and delignification bleaching processes. The terms “kraft pulps” and “chemical pulps” refer to pulps containing fibers that have been digested according to one or more of the sulphate, sulphite, soda, or organosolv processes, or mixtures thereof, and to recycled pulps, which may be a mixture of chemical and mechanical pulps. The pulp fibers may be of hardwood or softwood.

[0028] The inventive process can be effectively employed immediately after the chemical cooking or later in the pulp bleaching sequence. In fact, the inventive process can be used with a myriad of peroxide-based bleaching sequences. Non-limiting examples of suitable sequences include: the QPQP* and AZQP* sequences disclosed in C. Gustavsson, K. Sjostrom, W. Wafa Al-Dajani, “Influence of Cooking Conditions on the Bleachability and Chemical Structure of Kraft Pulps,” Nordic Pulp and Paper Research Journal, Vol. 14, No. 1, pp. 71-81 (1999), incorporated herein by reference in its entirety; the (D(EOP)DP, D(EOP)PD and XD(EOP)DnD sequences disclosed in Y. Sun, “Kraft Bleach Plant ECF Conversion: Comparison of Sequences involving Enzymes, Chlorine Dioxide, Oxygen and Hydrogen Peroxide,” Appita Journal, Vol. 52, No. 1, pp. 45-50 (January 1999), incorporated herein by reference in its entirety; the OD(EPO)DP, O(Z/D)(EPO)DP, (D/C)(EPO)DED, (D/Z)(EPO)DED, OD(EPO)DD, D(EPO)DED, and OD(EPO)D sequences disclosed in C. Courchene, B. Khandelwal, V. Magnotta, T. McDonough, A. Ragauskas and A. Shaket, “Comparative Evaluation of Low-AOX Hardwood Kraft Pulp Bleaching Sequences,” Proceedings of the 85th Annual Meeting of the Pulp and Paper Technical Association of Canada, Part B, pp. B307-B314, (Jan. 28-29, 1999), incorporated herein by reference in its entirety; and the D(EOP)DPP, D(EOP)PDP, DEDP, and DEPD sequences disclosed in P. Froass, J. Hamilton, A. Ragauskas, J. Sealey, and D. Senior, “Interaction of Hydrogen Peroxide and Chlorine Dioxide Stages in ECF Bleaching,” TAPPI Journal, Vol. 81, No. 6, pp. 170-178 (June 1998), incorporated herein by reference in its entirety. In all of the above-mentioned exemplary sequences, the inventive process should improve the performance of the EOP and P-stages (or P*-stage). Performance of the P-stage should also be improved where elevated temperatures and pressures are employed, as is disclosed in A. Audet, R. Berry, B. Roy, B. van Lierop, “High Temperature Alkaline Peroxide Bleaching of Kraft Pulps,” Int'l. Non-Chlorine Bleaching Conference Proceedings, Orlando Fla., (Pulp & Paper and Emerging Technology Transfer Inc.) Session Advances in ECF/TCF Technologies-Part 2, Paper No. 3-3, p. 6 (Mar. 24-28, 1996), incorporated by reference herein in its entirety.

[0029] Suitable pulps for the inventive process include softwood pulps with an initial Kappa number between about 5 and about 40, and hardwood pulps with an initial Kappa number between about 5 and 25. Higher Kappa number pulps could be treated as well but peroxide costs could be prohibitive. The term “Kappa number” is a well known term in the art and is calculated to provide a measure of the lignin content of pulp. Preferably, the initial Kappa number of the softwood pulp is from about 20 to about 35; more preferably from about 25 to 30; most preferably from about 28-30. Preferably, the initial Kappa number of the hardwood pulp is from about 10 to about 25; more preferably from about 15 to about 25 and most preferably from about 20 to 25.

[0030] In the processing of pulp suspensions, the term “consistency” refers to the percentage of pulp in the total mass of suspension. Suitable pulps for the process of the present invention include pulps having a consistency of from between about 3% and about 25%; more preferably from about 10% to about 20%; and still more preferably from about 10% to about 15%. Most preferably, the pulp has a consistency of about 10%.

[0031] The process of the present invention comprises bleaching pulp with at least one oxidizing agent to facilitate delignification of the pulp. Suitable oxidizing agents may include but are not limited to hydrogen peroxide, and other chlorine-free oxidizing compounds. Preferably, the oxidizing agent is a chlorine-free agent selected from oxygen, ozone, hydrogen peroxide, peracetic acid, peroxo monosulfate (also known as Caro's acid) or mixtures thereof. Most preferably, the oxidizing agent is hydrogen peroxide.

[0032] The process of the present invention also comprises use of at least one alkaline agent during the inventive bleaching stage to facilitate lignin removal. According to the process of the present invention, the alkaline agent can be any alkaline agent suitable for use in pulp bleaching processes. Preferably, the alkaline agent is selected from NaOH, KOH, Na₂CO₃, Mg(OH)₂, and Ca(OH)₂. Most preferably, the alkaline agent is NaOH.

[0033] Bleaching reaction rates can be significantly affected by pH, consequently, alkali or acid may be required to achieve optimal pH. Preferably, the pH of the process of the present invention is from about 9 to about 12.5; more preferably from about 11 to 12.5. Most preferably, the pH is about 12.2.

[0034] Temperature and pulp residence time are also important conditions in the inventive bleaching process. Suitably, the temperature of the present invention can range from about 60° C. to about 90° C. Preferably, the temperature of the present invention can range from about 70° C. to about 90° C. Most preferably, the temperature of the present invention is about 70° C. Usually, hydrogen peroxide is performed at medium consistency (8-15%). The process of the present invention can be batch or continuous. Residence time for the inventive bleaching process suitably can range from about 60 minutes to about 180 minutes. Preferably, the residence time ranges from about 90 minutes to about 180 minutes. Most preferably, the residence time is about 90 minutes.

[0035] The process of the present invention also comprises a novel use of at least one metal substituted xerogel to enhance the bleaching process. Suitable metals for substitution into the xerogel include, but are not limited to Al, V, W, Mn, Li, and Mo, and mixtures thereof. Preferably, the metal substituted xerogel (“Xgel”) is an alkyl metal substituted xerogel and is selected from Mn-Xgel, Al-Xgel, and W-Xgel. Most preferably, the alkyl metal substituted xerogel is Al-Xgel. Optionally, the metal substituted xerogel used in the process of the present invention may be a recycled metal-substituted xerogel.

[0036] The manner of xerogel preparation may impact the reactivity of the xerogel. Different types of metal adapted for xerogel preparation have a different effects on the reactivity of the xerogel in peroxide bleaching. Xerogel pore structure can affect catalytic activity and ion exchange capacity in hydrogen peroxide bleaching. Appropriate preparation conditions for synthesizing xerogels would be apparent to the skilled artisan.

[0037] The metal substituted xerogels used in the present invention can be prepared by processes known to those of skill in the art. Most preferably, the metal substituted xerogels are prepared according to the procedure described in R. Neumann, M. Chava, and M. Levin, “Hydrogen Peroxide Oxidations Catalysed by Metallosilicate Xerogels,” J. Chem. Soc. Che. Commum. pp. 1685-87 (1993), herein incorporated by reference in its entirety, and set forth in Example 1 of the present disclosure.

[0038] The oxidizing agent, alkaline agent, and metal-substituted xerogel used in the process of the present invention are provided in amounts effective to improve brightness and delignification of the bleached pulp. According to the present invention, improved brightness and delignification is achieved by increasing brightness and delignification above that present in the pulp prior to bleaching in accordance with the present invention. Preferably, the process of the present invention improves pulp TAPPI brightness from about 27.8 to about 37.8 and improves pulp delignification by from about 35% to about 38%. Most preferably, pulp brightness is improved by at least about 30% and pulp delignification is improved by at least about 38%.

[0039] The amount of oxidizing agent used in the process of the present invention is preferably from about 2% to about 4% by weight of oven dry pulp. Most preferably, the amount of oxidizing agent is about 2% by weight of oven dry pulp. The amount of alkaline agent is preferably from about 2% to about 4% by weight of oven dry pulp. Most preferably, the amount of alkaline agent is about 2% by weight of oven dry pulp. The amount of metal substituted xerogel is preferably from about 0.05% to about 2% by weight of oven dry pulp; more preferably from about 0.05% to about 0.2%. Most preferably, the amount of metal substituted xerogel is about 0.2% by weight of oven dry pulp. While various combinations of oxidizing agent, alkaline agent and metal substituted xerogel may be used, the process of the present invention most preferably includes a combination of hydrogen peroxide, NaOH, and Mn-Xgel. These components can be added simultaneously or sequentially, however, if added sequentially, the oxidizing agent is preferably added last.

[0040] Manganese and other metallic ions present in kraft pulps can have a particularly adverse effect on the bleaching efficiency of chlorine-free bleaching agents such as ozone and alkaline peroxide compounds. Thus a chelating treatment step, in which a compound is added to form complexes with metallic ions, may be useful prior to an alkaline peroxide bleaching step. Suitable chelating agents can include but are not limited to nitrogenous organic compounds, polycarboxylic acids, or phosphonic acids. Preferably, the chelating agent is selected from ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA), oxalic acid, citric acid, hydroxyethylene diaminetetraacetic acid (HEDTA), diethylenetriaminepentamethylene-phosphonic acid (DTPMPA), or tartaric acid. Most preferably, the chelating agent is EDTA.

[0041] The type and added amount of chelating agent depends on the type and amount of trace metal ions in the incoming pulp, as well as the conditions in the treatment such as pH, temperature and residence time. Preferably, the amount of chelating agent added is in the range of from about 0.3% to about 0.8% by weight of dry pulp, calculated as 100% chelating agent, and most preferably about 0.6% by weight of oven dry pulp.

[0042] After alkaline peroxide delignification, the pulp can be used for direct production of paper. Alternatively, the pulp may be finally bleached to a desired higher brightness in one or more stages. Final bleaching may be carried out by use of the chlorine free bleaching agents indicated above, including additional inventive metal substituted xerogel-catalyzed alkaline peroxide stages, with optional intermediate washing or extraction stages, or by use of chlorine containing bleaching agents, such as chlorine dioxide.

[0043] The invention is further discussed in conjunction with the following examples, which are merely illustrative of the present invention.

EXAMPLES

Example 1

[0044] To test the effectiveness of metal substituted xerogels in catalyzing an alkaline hydrogen peroxide bleaching stage, the following experiments were performed. SW kraft pulp (Brownstock, initial Kappa number=28.5) was bleached, with and without a metal substituted xerogel, using 2.0% hydrogen peroxide and 1.0% NaOH. The control hydrogen peroxide bleaching experiment employed no xerogel but included 0.20% MgSO₄. Control hydrogen peroxide bleaching experiments were also conducted using 0.2% charges of the alkyl metals Ti(OiPr)₄, Mn(OEt)₂, V(OiPr)₄, Mo(OEt)₄, W(OiPr)₄, Li(OEt), and Al(OEt)₃, (where OEt=CH₂CH₃O and OiPr=CH(CH₃)₂O) respectively, and test experiments were conducted with 0.2% charges of the following alkyl metal substituted xerogels, Ti-Xgel, Mn-Xgel, V-Xgel, Mo-Xgel, W-Xgel, Li-Xgel, and Al-Xgel, respectively. A control experiment using only alkyl metal without prepared xerogel was also performed. All of the experiments were performed at 10.0% consistency and a temperature of 70° C., for 90 minutes.

[0045] The improvements in delignification are summarized in Table I and FIG. 1. A comparison of the delignification data indicates that the usage of 0.2% tungsten, lithium, or aluminum ethoxide substituted xerogels in an alkaline peroxide stage improves delignification by 44, 37, and 38% respectively. This extent of delignification is greater than what can be achieved with 4% hydrogen peroxide under comparable bleaching conditions as summarized in Table 2. Also, the additional control studies, with alkyl metals (no xerogel) and xerogel only, failed to yield improved peroxide delignification of SW kraft pulps, as shown in Table 1. These latter experiments demonstrate that the improved peroxide bleaching properties observed with metal substituted xerogels is due to the chemical composition of the catalyst and not to the individual components. 1

[0046] 2

[0047] The metal xerogels used in the above and below-mentioned experiments were prepared in the following manner: silicone tetraethoxide (57 mmol) was dissolved in absolute ethanol (17 ml) and 0.15 N HCl aqueous solution (0.11 ml). The mixture was heated to 60° C. for 90 minutes and then cooled to room temperature followed by addition with stirring of 3.00 mmol of the appropriate metal alkoxide (i.e., metal=Ti, Mn, V, Mo, W, Li, and Al, respectively). The mixture formed a gel immediately and was left in an open-beaker allowing slow evaporation of solvent. A solid was formed within 12 -24 hours. The solidified xerogel was ground and dried at 100° C. for 12 hours. The improved delignification accomplished with the metal substituted xerogels also yields improved pulp brightness values as summarized in Table 3. All metal substituted xerogels (i.e., metal=Ti, Mn, V, Mo, W, Li, and Al, respectively) improved pulp brightness properties with the tungsten, lithium, and aluminum substituted xerogels being the most effective, providing brightness gains of +30%. Control studies using 2% hydrogen peroxide indicated that the brightness gains achieved with 0.2% W, Li, or Al substituted xerogels was comparable to what could be achieved with 4% hydrogen peroxide. In the presence of hydrogen peroxide, the metal substituted xerogels may act as catalysts leading to the formation of inorganic peroxides and/or chelate unwanted metals present in the pulp. 3

[0048] These experiments clearly show that the use of alkyl metal xerogels with an alkaline peroxide stage improves the delignification and brightening properties of a peroxide bleaching stage for kraft pulps.

Example 2

[0049] Experiments were also conducted to determine if a metal xerogel-catalyzed peroxide bleaching treatment had an detrimental effects on pulp strength properties. SW kraft pulp (Brownstock, initial Kappa number=21.2) was bleached, with and without a metal substituted xerogel, using 2.0% hydrogen peroxide and 2.0% NaOH. The control hydrogen peroxide bleaching experiment employed no xerogel but included 0.30% MgSO₄ (This salt was not added to the xerogel experiments). Test experiments were conducted with 0.05%, 0.2%, and 0.3%, respectively, charges of the following alkyl metal substituted xerogels, Mn-Xgel, W-Xgel,and Al-Xgel, respectively. All of the experiments were performed at 10.0% consistency and a temperature of 70.0° C., for 90 minutes. After bleaching, the pulps were washed with deionized water (it is not necessary that deionized water be used) and physical properties were determined using standard TAPPI T220 om-88 “Physical Testing of Pulp Handsheets” testing procedures.

[0050] FIG. 2 summarizes the observed changes in delignification for the pulp bleached with Al, W, and Mn xerogels (xerogels prepared as discussed above example 1). Table 4 summarizes the physical properties of the bleached pulps. 4

[0051] All pulps are refined by PFI (revs. 4000). Pulp refining is a mechanical treatment of pulp fibers. This process increases the strength by increasing the surface area and improves the capacity of the pulp to absorb water. The treatment is described in C. J. Biermann, Essentials of Pulping and Paper-making, Academic Press, p. 137, (1993), incorporated herein by reference in its entirety.

[0052] The bleaching and physical property results summarized in FIG. 2 and Table 4 indicate that the metal xerogels improve delignification of a peroxide stage while yielding peroxide bleached pulps that exhibit equal or better physical properties to the control pulp bleached only with hydrogen peroxide.

Example 3

[0053] Experiments were also conducted to determine the effect of a Q (chelating) stage on a metal substituted xerogel peroxide stage. SW kraft pulp was chelated with EDTA at a pH of 5, for 30 minutes, at a pulp consistency of 2%. The consistency on pulp bleaching is usually calculated as the weight percent of pulp in given pulp suspension (CSC %=od pulp wt/wet pulp wt* 100) The required consistency is easily obtained by adjusting the amount of water in the pulp suspension. Sulfuric acid was used to adjust pH. The peroxide bleaching experiments of Example 2 were then repeated using a 2.0% charge of hydrogen peroxide at 10.0% consistency and a temperature of 70° C., for 90 minutes. 5

[0054] The results of the peroxide delignification using a chelated pulp with and without metal substituted xerogels are summarized in FIG. 3 and Table 5. These results indicate that the tungsten, aluminum and manganese xerogels still enhance delignification, although the former two operate significantly better.

[0055] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.